The costs associated with transportation of important minerals such as coal often comprise a substantial proportion of the total costs of material handling. Conventional transportation systems such as belt conveyors, road and rail are generally inefficient and make considerable demands on space and resources. In many circumstances, transportation costs are often higher than the actual mining costs and usually represent the principal cost at the point of consumption.
As an alternative to these conventional methods, attempts have been made to transport minerals in suspension in slurry form along a pipeline. However, such slurry systems have not generally gained acceptance as a commercially viable means of mineral transportation, largely due to a lack of development in a number of associated areas of technology. In particular, it has been found that pump pulsations, inadequate valve performance in handling larger particles, problems associated with slurry preparation, and inadequate theoretical understanding of the dynamic behavioural characteristics of the slurry have resulted in slurry transportation systems not yielding the economic motivation for widespread commercialisation.
In an attempt to ameliorate these problems, a so called rotary ram pump was developed and this is described in detail in Australian patent specification 461204. The object of that invention was to provide the advantages of a positive displacement pump while at the same time attempting to provide substantially uniform delivery characteristics. However, in practice this pump has been found to be inadequate in several respects.
It has been found that a major problem with the known rotary ram pumps is the tendency for larger particles to block the partially open ports on the suction side of the pump during the initial inlet stage of the suction cycle. This problem is particularly significant when pumping coarse (dense phase) coal slurry due to the presence of relatively large size coal particles. Once the ports become even partially blocked, the smooth fluid flow through the pump is disrupted thereby sending sharp pressure pulses down the pipe line and significantly reducing the effective suction pressure. It has been found through extensive research and development that even minor disturbances of this nature can cause transient instabilities in the sensitive slurry bed which in turn can result in blockages in the hydraulic transport line where pulseless conditions must be maintained within relatively narrow limits in order to permit efficient and smooth pumping of the slurry. This problem is further exacerbated when eratic dynamic behaviour of the pump resulting from partial port blockages causes the free-floating pistons to collide with their respective end stops. This sends relatively severe shock pulses down the hydraulic transport line, further destabilising the vibration sensitive slurry bed and resulting in line blockages which often need to be manually cleared.
In addition, it has been found that the partial port blockages of the type discussed can create a filtration effect, preferentially allowing the passage of finer particles, whilst substantially impending the movement of relatively coarse particles through the pump. This tends to exacerbate the intial blockage, and can eventually form a "plug" in the port, substantially impairing the efficient performance and operation of the pump. Moreover, this disturbs the uniformity of particle size distribution, which has found to be essential for pumping slurries over longer distances.
It is therefore an object of the present invention to provide an improved pump which overcomes or substantially ameliorates at least some of these disadvantages of the prior art.